The invention relates to a method and device for multiple-stage arc welding, whereby a bolt having a flange at the end that is to be welded on is moved up to a metal structure and welded on.
Welding a metal bolt to a metal structure by means of arc welding is a widely used method, which finds application especially in the automobile industry. There are essentially different welding methods with corresponding designs of the associated bolts.
In one welding method, the so-called lifting ignition or electric-arc bolt welding method, the end of the bolt to be welded on has a flange that on its underside usually also has a bead of material that may be domed or has the shape of a blunt cone. In the lifting ignition method, the bead of the bolt is first moved up to a structure until an electrical contact is made. A high electric short circuit current from a welding power source is sent through the bolt and the structure. The bolt is then again slightly lifted from the structure, whereby an electric arc is produced between the bolt and the structure that fuses the structure in a fusion region as well as parts of the bead. After a specified period the bolt is lowered into the fused mass produced in the fusion region, whereby on subsequent cooling a very sturdy weld joint is produced between the bolt and the structure.
Another method for arc welding bolts to a metal structure is the so-called tip ignition method. There the end of the bolt to be welded on has a salient or projecting tip, which is first moved up to the structure. The tip is dimensioned in such a way that when a welding current is applied it is suddenly vaporized with formation of a hot plasma, whereby the remaining end to be welded on and the structure are fused. At the same time, the bolt is rapidly lowered into the fused mass. The great energy required for this operation results in a very loud explosive noise. In addition, the structure generally must be supported on the back of the fusion region, since otherwise the impact of the bolt at high speed may cause vibrations that have an adverse effect on weld quality. Besides, the quality of the weld is greatly dependent on the quality of the tip.
The increasingly broader application of bolt welding methods also results in applications that cannot be accomplished particularly well by either of these two methods. Thus, there are cases in which the structure in the fusion region is coated or fouled, especially with a wax layer or oil film, and cases in which the structure is a material, particularly aluminum, having an oxide layer. Coatings deleterious to the welding operation, such as for example hot galvanizing, are also encountered. Combinations of these cases likewise occur.
It is well known that two stages may be provided in bolt welding, the first stage acting to establish the surface condition and/or being a cleaning stage and the second stage producing the weld joint. However, a reproducibly great cleaned surface cannot always be produced in a single cleaning stage, since in variable surface conditions, the first ignition of an electric arc plainly leads to results that are hard to reproduce.
For such cases, the object of the present invention is to make available a generic method and a device for arc welding a bolt to a metal structure, which permits high-quality and reproducible weld joints on surfaces that are provided with an oxide layer and/or some other surface coating, in particular oil, wax or even a zinc layer, where the method should also work under acceptable operating conditions and be economical.
One method of the present invention uses the lifting ignition method wherein the bolt to be welded has a flange is moved up to the metal structure and, prior to being welded thereto, the surface of the structure will be cleaned in a two-step process by an electric arc cleaning the surface of the structure with the bolt off the surface. The strength and duration of the current used for cleaning may be varied as well as the distance between the bolt and the surface. Current strength may be in the range of between 100 MA to 100 A; the duration in the range of 40 MS to 150 MS; and the distance may be in the range of 1.0 to 3.0 mm. The structure of the device that practices the method may include a welding source having two capacitor batteries, a bolt holder, a linear drive, such as an electromagnetic linear motor, for moving the bolt holder and a rapid control to control the course of motion and the accuracy of the current within the range of 2 to 20 ms.
More specifically, the method consists of using an arc generated between the bolt and the structure, in the fusion region therebetween to clean the surface of the structure in the fusion region in at least two cleaning steps, and only then is the metallic connection between the bolt and the structure made by the customary electric-arc bolt welding.
When the distance of the bolt from the structure and the operating time of the electric arc are correctly coordinated, the arc may in particular be used for cleaning the structure surface of an oil film or wax layer, as well as of oxides or hot galvanizing. In this way, thorough cleaning of the structure surface can be obtained in at least the fusion region of the bolt, so that the subsequent metallic connection can be reliably obtained by electric-arc bolt welding without special effort. There the first cleaning step by ignition of a high-power but narrowly limited electric arc produces a precise contact area, which upon renewed lowering of the bolt and its lifting permits a highly reproducible second cleaning arc that then cleans the entire later fusion region.
According to an advantageous development, cleaning of the structure surface therefore is carried out in two separate successive cleaning stages where control of the entire multiple-stage arc-welding method is effected by moving the bolt in the direction of the structure and away from it, as well as by the time and duration of the electrical connection of the bolt with the welding current source. Different aspects of the cleaning operation may be focused on in this two-stage cleaning by means of the electric arc.
In particular, if the method in the first cleaning stage is produced after the bolt which had been in contact with the surface is subsequently moved away, an especially intensive cleaning of a relatively small surface is obtained, which corresponds approximately to part of the cross-sectional flange area of the bolt. In this method stage, especially oil films and oxides are satisfactorily removed.
After completion of the first cleaning stage the bolt remains raised from the structure, and the electric arc remains disconnected. Accordingly, the two cleaning stages are completely separated from one another, and excessive heating of the bolt and structure is also avoided, so that upon renewed lowering of the bolt, it does not encounter a molten surface. Due to the relatively great distance of the bolt from the structure surface, operation of the method in the second cleaning stage is produced by again moving the bolt up to contact the surface of the structure and, after being connected to the welding current source, is moved away a predetermined distance which results in an electric arc that causes a greater area of the surface to be cleaned; this area corresponds to approximately the size of the cross-sectional flange area of the bolt. This cleaning stage is especially suitable for also removing wax layers from the structure surface, where these wax layers may be present in a thickness of 1 to 10 g/m2. In this way, all prerequisites for subsequent metallic connection by electric-arc bolt welding are present. In the second cleaning stage, the distance of the bolt from the structure is predetermined, or selected, such that a sufficiently great region of the structure can be cleaned. The most favorable distance therefore depends approximately on the size of the later fusion region, i.e., on the diameter of the flange to be welded on.
Since fusion of the structure surface must take place at this point anyway, the second cleaning stage leads smoothly into the operation of electric-arc bolt welding; i.e., the arc remains uninterrupted in these two method stages.
Only by correct operation of the electric arc, necessary in every case, is it thus possible first to carry out thorough cleaning of the crucial regions of the structure surface and then to obtain an especially high-grade metallic connection by the method according to the invention.
When the materials are the same, i.e. both the bolt and the structure are either aluminum or steel or the like, then the method according to the present invention is especially well implemented and acts to produce a solid weld therebetween.
Performance of one or more additional cleaning stages is possible and may be of advantage for certain applications.
According to an especially advantageous refinement of the invention, before cleaning a characteristic magnitude that is proportional to the degree of fouling of the surfaces to be welded together, by means of which the number of cleaning stages is determined, is first measured. This is used to determine the degree of required cleaning and insufficient or excessive cleaning of surfaces is avoided. The electrical surface resistance, which in first approximation is proportional to the fouling of the surfaces, is the obvious characteristic magnitude. The surface resistance and hence the required number of cleaning stages is determined by means of a current and/or voltage measurement between the surfaces to be welded together. In a special refinement, the contact voltage between the bolt and the structure is measured.
In an advantageous development of the method according to the invention, cleaning is omitted if the characteristic magnitude falls below a first threshold value, takes place once if the characteristic magnitude lies between the first threshold value and a second threshold value, and takes place at least twice if the characteristic magnitude lies above the second threshold value. Criteria for the number of cleaning stages, which may differ depending on the type of weld to be obtained, the nature of the surface, the materials to be welded that are used and other welding-specific parameters, are specified by means of at least one, preferably two or more, threshold values for the characteristic magnitude.
A device for performance of the method must be suitable for making available even brief and exact fairly strong current surges; the necessary welding current for the electric arc must be available shortly thereafter. This requires a welding current source with suitable control means, which may for example contain one or two capacitor batteries. In addition, the device must have all typical means for performing the lift ignition method, namely a bolt holder, a linear drive for moving the bolt holder and a rapid control capable of controlling the course of motion of the bolt and the response of the welding current with great accuracy, in particular with time accuracies in the range of 2 to 20 ms.
A linear drive, in particular with an electromagnetic drive, has proved to be especially suitable for the present task. Such drives, as described for example in U.S. Pat. No. 6,215,085, permit precise control or regulation of the position and speed of the bolt during the welding operation.
Accordingly, it is an object of the present invention to provide a method and device for multiple-stage welding which overcomes the disadvantages of the prior art and provides reliable and sturdy weld connections between the bolt and the structure. It is another object of the present invention to use the lift ignition method.
It is still another object of the present invention to provide a method and device for multiple-stage welding that cleans the surfaces to be welded prior to producing the weld, which is relatively inexpensive, which is simple to practice and reliable to produce.